Cooperative MIMO (multiple input multiple output) utilizes distributed antennas which encompasses a multitude of cells and sites. On the terminal (UE, user equipment) side, only antennas belonging to the terminal are used. Cooperation improves the performance of a mobile system by exploiting the benefits arising by the use of multiple antennas on different locations. These are diversity, joint transmission, interference cancellation, multi user multiplexing and beam forming. Cooperative concepts are envisaged as an important element to improve the cell edge user experience in frequency reuse 1 layouts, in which the same frequency band is used in adjacent cells/sectors.
The concept of cooperation works best, if the cooperation area (CA) is chosen so large, that all of an UEs strong base stations (BSs)—especially contributors to the cell edge interference—are part of the CA which serves that UE. This is the so called “user centric assignment”. Since each UE has an individual set of strongest BSs, fixed and pre-configured CAs can not meet these challenges.
There is a simple trade-off:
Full cooperation is the optimal solution, but not realizable. Hence, the CA must be limited, but as soon as limited CAs are introduced, an interference between them is introduced.
A solution is the already mentioned user centric assignment. This, however, changes the problem into a very low penetration rate and the user centric assignment is hard to realize because of the probability to find a group of UEs with an identical set of strongest BSs and additionally identical to an existing CA is very low.
Documents [1], [2] and [3] mentioned below describe a partial cooperative multi point (partial CoMP, pCoMP) framework.    Document 1: “Wolfgang Mennerich and Wolfgang Zirwas. Implementation Issues of the Partial CoMP Concept. In Proceedings of 21st Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2010), Istanbul, Turkey, September 2010”.    Document 2: “Wolfgang Mennerich and Wolfgang Zirwas. User Centric Coordinated Multi Point Transmission. In Proceedings of IEEE 72nd Vehicular Technology Conference (VTC2010-Fall), Ottawa, Canada, September 2010”.    Document 3: “Wolfgang Mennerich and Wolfgang Zirwas. User Centric Scheduling in Cooperative Networks. In Proceedings of IEEE Middle East Conference on Antennas and Propagation (MECAP 2010), Cairo, Egypt, September 2010”
The partial CoMP framework combines huge and overlapping CAs with partial channel reporting to avoid CA-edge UEs and to increase the penetration rate of user centric served UEs drastically with respect to keep the system realizable.
Assignment between UEs and CAs is generally an open issue in CoMP scenarios if it should be done user centric. The challenge is to find CAs which are best fitting to user groups (UGs), that are formed depending on the (pre)defined CAs.
UE-CA assignment is typically done centralized and in a serial way, i.e. an assigned UE(m) influences the conditions for the assignment of UE(m+1), but not the other way around.
However, this approach has several disadvantages.
For example, the optimal CAs might become very huge. It has been observed in simulations using the 3GPP-SCME model (3GPP-SCME, 3rd Generation Partnership Project Spatial Channel Model Extended), that the strongest BSs of an UE can be distributed over several cell sites. Hence, a back-haul network must be installed between all possible members of a CA, but only a part of it will be used later. This, however, leads to a waste of resources.
A further drawback of this approach is that an enormous amount of radio resources is needed for channel reporting because UEs which could be connected to several CAs must report their connectivity values to all of these CAs.
Moreover, if a CA gets to many connect-requests, it must decide which UEs will be served and which not. This is typically negotiated between CAs on the network back-haul.